$B)$ Nitrogen is $Gr$. 15thelement. it has half filled $2 p$ orbital. $(He)\,2 s^2\, 2 p^3 \,(He) \,2 s^2 \,2 p^3$
$C)$ Oxygen is $Gr$, $16th$ element. $(He) \,2 s^2\, 2 p^4$
$D)$ Radium is $Gr,$ $2nd$ element. $( R n) \,7 s^2$
The attachment of electron is most difficult for Radon.